Optical encoder



June 11, 1968 J. FISHER 3,388,392

OPTICAL ENCODER Original Filed Sept. 16, 1960 4 Sheets-Sheet l INVENTOR. Jaw/v [is/v0? 7 'BY% 4 e;

June 11, 1968 J. FISHER OPTICAL ENCODER 4 Sheets-Sheet 2 Original FiledSept. 16, 1960 INVENTOR. 7 JOHN fiS/lff? BYMI; A9, W I I ATTORNEYS J.FISHER OPTICAL ENCODER June 11,

4 Sheets-Sheet 3 Original Filed Sept. 16, 1960 R a T H N7 mm, a W m I NM A J,

June 11, 1968 J. FISHER 3,388,392

OPTICAL ENCODER Original Filed Sept. 15, 1960 4 Sheets-Sheet 4.

, INVEN TOR. Jowv /5HE/? A 7- Toms/75 United, States Patent Oflice3,388,392 Patented June 11, 1968 14 Claims. or. 340-347 The presentinvention relates to encoders and particularly to optical encoders fordigitally indicating shaft position.

This application is a continuation of my copending application Ser. No.56,590, filed on Sept. 16, 1960, now abandoned.

An important object of the present invention is to provide a new and animproved encoder for digitally indicating shaft position, which encoderis highly accurate and relatively free from ambiguity but, yet, iscompact in structure so as to accommodate its use in locations wherespace is at a premium.

A further object of the present invention is to provide a new and animproved optical encoder for converting shaft position into a numbercomprised of a plurality of binary digits, the encoder including a codewheel having a plurality of radially displaced zones extendingcircularly about the code wheel with each zone being comprised ofalternate transparent and opaque areas and a light-responsive readerunit for reading each of the zones with the encoder being so constructedand arranged that the electrical outputs of the reader units for eachzone are substantially the same when the units are subjected to maximumillumination for the corresponding zone.

It is also an object of the present invention to provide a code wheelfor indicating shaft position in the Gray, or reflected binary, codewith the code wheel having a digit value code zone for each digit of theGray number except for the two digits of the Gray code which arerepresentable by the two coarsest digit zones and which have the samenumber of divisions but which are angularly displaced from each other,these digits being represented by a single zone read by a reader whichis angularly displaced.

In the above objects and in the following description and claims theword transparent area is intended to cover an area which transmits lightand includes an open slit in an otherwise opaque wheel.

In accordance with the preferred embodiment of the present invention,the possibility of obtaining a false or an ambiguous reading from a codewheel having a plurality of circularly extending digit zone tracksthereon radially displaced from each other and read by lightrcsponsivereader units disposed on one side of the wheel and responsive to lightfrom a light source on the other side of the wheel disposed inwardly ofthe zone tracks and which illuminates the reader units through thetransparent areas of the code wheel as they rotate past the units isminimized by providing a mask adjacent the code wheel with the maskhaving slits which correspond to respective ones of the zones to controlthe amount of light which passes from the light source through thetransparent areas to the photocells and to provide more definiteresolution as the zone being read by the reader unit changes from atransparent to an opaque area. The slits for the zones have differentareas, with the areas of the slits being such that substantially equalquantities of light are transmitted by the slits when subject to fullillumination. I have found that there is a point as one proceedsinwardly toward the light source, where the areas of the slits must beincreased as one moves closer to the light source, rather than decreasedas one would expect,

to pass a given quantity of light. In other words, a sli of given areaand defining a given are, once inside this point, will pass less lightas it is moved inwardly with the area and are maintained constant. Inthe preferred embodiment, the slits are made as wide as possibl in acircumferential direction with all slits su'btending the same angulararc wherever possible and the radial extents of the slits for the zonesvaried to provide the area necessary for each slit to transmit the sameamount of light as sensed by the reading units. The radial extents ofthe corresponding zones vary in accordance with the radial extents ofthe slits. Where the slits all subtend the same angular arc or increasein circumferential extent as one moves outwardly, the radial extent ofthe slits and zones can be made to decrease. In the preferred andillustrated embodiment, the angular arc subtended by the slit of thefine zone of the wheel is about half that of the angular arc subtendedby the other slits of the encoder. In addition to the foregoing, twolight responsive elements and corresponding slits are preferablyprovided for reading each zone with one photocell going dark as theother photocell goes light, with the outputs of the photocells beingcombined to provide the output signal of the reader unit for the zone.The construction as described, in addition to minimizing false andambiguous readings, facilitates the provision of a compact wheel sincethe use of slits which have a greater circumferential extent as oneproceeds away from the light source eliminates the need for extendingthe radial extent of the slits, and in turn of the zones to acquire awheel which has reader units subjected to substantially equal amounts oflight. The compactness of the wheel is further facilitated byinterchanging zones of the .wheel that when proceeding outwardly fromthe center of the wheel, the zones do not progressively increase indivisions or resolution. In the preferred and illustrated embodiment,the wheel is coded in the Gray code where the successive digits of thecode are respectively represented by zones which have 2 2 2 2 2divisions, and the zones which would normally appear as the third andfourth digit zones of the code wheel proceeding out wardly from thecenter of the'wheel, are interchanged so that they are positionedbetween the zones representing the sixth and seventh digits.Furthermore, the first and second digits are represented by a singlezone with the single zone being read by two reader units angularlydisplaced from each other.

The elimination of false and ambiguous readings is also facilitated bysupporting the light-responsive elements of the reader units in openingsin the support member or members therefor with the openings extendingalong the path of light from the light source, preferably taking intoconsideration the refraction of light when the Wheel or mask or both areof glass through the code wheel and the corresponding slits. Thisconstruction faciiitates the reception of the maximum amount of light bythe reader unit.

Further objects and advantages of the present invention will be apparentfrom the following detailed description made with reference to theaccompanying drawings forming a part of the present specification and inwhich:

FIG. 1 is a longitudinal sectional view of an encoder constructed inaccordance with the present invention;

FIG. 2 is a view of the encoding wheel looking at the face of theencoding wheel;

FIG. 3 is a sectional view taken approximately along line 3-3 of FIG. 1;

FIG. 3A is a schematic View showing certain of the slits of the mask ofFIG. 3 rotated from their positions shown in FIG. 3; and

FIG. 4 is a fragmentary sectional view of part of FIG. 1 on an enlargedscale.

Referring to the drawings, FIG. 1 is a sectional view through an encoderembodying the present invention. The encoder includes a code wheel 10,for indicating shaft position, fixed to a shaft 11 with the code wheelbeing coded in the Gray code. The shaft 11 is rotatably supported in abore in an elongated support member 15 by ball bearings 12, .13, themember 15 having an enlarged part 16 at the end thereof adjacent to thewheel 10. The ball bearings 12, 13 are positioned in a bore 13 extendingthrough the member 15 and the shaft 11 extends outwardly of each end ofthe bore 18, the portion of the shaft 11 extending outwardly of theright-hand end of the bore 18 adjacent to the enlarged part 16 havingthe code Wheel secured thereto for rotation therewith. The supportmember is pressed into a bore 20 in the bottom of a cup-shaped casting21 and the enlarged part 16 has a peripheral portion 22 which extendsoutwardly from the periphery of the enlarged part 16 to close the openend of the casting 21.

The code wheel 10 is preferably coded in the Gray, or reflected binary,code. This code is described in detail in Gray Patent No. 2,632,058 andis well understood by those skilled in the art. The code of the codeWheel is best shown in FIG. 2 and, as shown therein, comprises aplurality of concentric code zones 25, 26, 27, 28, 29, 3t), 31, 32, 33,each zone extending circularly about the code wheel with the zones beingradially displaced from each other and with each zone being comprised ofa different number of alternate transparent and opaque areas,respectively designated by the reference characters T and 0 representingdivisions of the zone.

In the conventional Gray code, the two coarsest, i.e., the 2 digits, arenormally represented by zones each of which have two divisions extending180 with the code zones being displaced from each other by 90, and eachsuccessive digit of the number is represented by a zone having a numberof divisions represented by 2 Consequently, the third digit zone has anumber of divisions in the zone corresponding to 2 the fourth digitzone, 2 the fifth digit zone, 2 and the sixth digit zone 2 the seventhdigit zone, 2 the eighth digit zone, 2"; and the ninth digit zone, 2Conventionally, each digit is represented by a separate code zone on thedisk, with the two coarsest digits being represented by separate codezones having opaque and transparent areas displaced by 90 from the zeroreading line of the zone. In the illustrated disk, the two coarsestdigits are combined into one zone but are read by two separate readingheads, as described in more detail hereinafter. Furthermore, in theconventional Gray code wheel, the zones are arranged in the order oftheir number of divisions proceeding radially outwardly from the centerof the wheel. In the disclosed and described code wheel, the third andfourth digit zones having 2 and 2 divisions therein are disposedintermediate the zones 29 and 32 for the eighth and ninth digits,respectively. The interchanging of the zones of the code wheel in themanner described facilitates the provision of a compact wheel, aspointed out hereinafter.

Each of the zones is read by a reader unit comprising, in theillustrated embodiment, a pair of photocells 35, 35 for each code zoneof the wheel. Since the two coarsest digits of the code are representedby one code zone on the code wheel, the zone 25 is provided with twopairs of photocells 35, 36, on of the sets of the photocells 35, 36having a prime appended thereto to indicate the reader unit for onezone, with the other pair being designated by the reference numerals 35,36 to indicate a reader unit for the other zone combined into the onezone 25. The photocells 35, 36 for reading each zone are displaced fromeach other so that when one cell is at the center of an opaque division,the other cell is at the center of a transparent division. Consequently,when one cell is going dark as an area is changing from transparent toopaque, the other cell is going light since the area opposite that cellwill be changing from opaque to transparent, and vice versa. Thephotocell of each zone is located on what is normally the zero readingline of the zone and it will be noted that the zones are angularlydisplaced so that the zero reading lines of the various zones are notaligned. The zero reading lines for each zone are indicated by adot-dash line designated by the same reference character as thecorresponding zone but with a z appended thereto.

The light for reading the code wheel is provided by a light source 40illustrated in the form of an incandescent lamp disposed on the side ofthe wheel 10 opposite the photocells 35, 36. The lamp 40 is mounted in acup-shaped holder 41, bolted or otherwise secured to the peripheralportion 22 of the enlarged part 16 and including a socket structure 42in the bottom of the holder for removably supporting and energizing thelamp 40. The center line of the lamp 40 is generally aligned with theaxis of the shaft 11 so that the filament 44 is disposed on the extendedaxis of the shaft 11.

The light rays emanating from the filament 44 will pass through thetransparent areas of the code wheel 10 to the photocells 35, 36 when atransparent area is opposite the photocells 35, 36. It will be apparentfrom a consideration of FIGS. 1 and 4 that the path of direct light raysfrom the filament through a transparent area of the disk to a photocellhas a different angle of inclination for the photocells of the differentcode zones. In other words, the light rays traveling from the filament44 to the photocells of the innermost zone have a path of oneinclination with respect to the axis of the shaft 11, while the lightrays traveling from the filament 44 to the photocells for reading theoutermost zone have a path which has a much greater angle of inclinationwith respect to the shaft axis. In accordance with one feature of thepresent invention, the photocells are mounted in the enlarged portion 16and are each received in an opening or bore which extends aiong the pathfor light from the filament through a trans parent area opposite to thephotocell so that the optical axis of the photocell lies along the pathof light to the cell. This provides the maximum response of thelightresponsive photocells to the light passed by the transparent areasof the wheel. While the photocells have been shown as mounted on asingle support member and while this is preferable to provide a compactwheel, in environments where a compact wheel is not a prime requisite,the photocells might be mounted on separate members having bores oropenings which extend along the direct light path from the cell to thefilament. It will be understood that where the wheel is of glass orother medium which refracts light, the path for light from source to thecells will be a refracted light path and the cell is positioned with itsoptical axis along the refracted path.

If ambiguities of the readings are to be prevented, it is important thatstray light does not destroy the accuracy of the reading by thephotoelectric cells. To provide improved definition, a mask is disposedbetween the code wheel 16 and the enlarged part 16 and is designated bythe reference numeral 5t). This mask could also be positionedimmediately in front of the wheel. The mask has a slit opposite each ofthe photo-cells and the slits have been given the same referencecharacter as the zones read by the corresponding photocells with an sappended thereto. It will be that the slits for the photocells forreading the coarsest zone 25 are designated by the reference character25s; While those slits disposed opp0- site the photocells for readingthe outermost zone are designated by the reference character 33s, etc.The mask 50 has an opening 51 therein through which the shaft 11 extendsand is clamped in position by a clamp block 52 which clamps the maskagainst a cooperating block 53 fixed to the enlarged part 16. It will beappreciated that a plurality of such pairs of clamp blocks is providedfor mounting the mask.

In accordance with the present invention, the slits are proportioned sothat with full illumination from the light source the slits will passsubstantially the same quantity of light. Although the intensity of thelight from the light source has an inverse relationship with respect tothe distance from the source, I have found that when slits arepositioned close to the axis, as in the preferred embodiment, a slit ofa given area will pass a quantity of light which has a directrelationship to its distance from the slit rather than an inverserelationship. Consequently, the slits 25s, 26s, 27s, 28s, 29s, actuallymay have to de crease in area in the order named to provide slits whichtransmit substantially equal quantities of light when subject to fullillumination. In the encoder which was constructed, the slits 25s-29sWere respectively positioned from the axis of the mask, and the areas ofthe slits decreased proceeding outwardly from the axis of the mask.

Preferably, for all zones where it is possible, the slits subtend thesame angular are as the slits 25s so that the same angular movement ofthe wheel is necessary to move the edge of an opaque or transparent areafrom one side or" the slit to the other and the area is adjusted byvarying the radial extent of the slits. Referring to FIG. 3, it will beapparent that the slits decrease in radial extent proceeding outwardlyfrom the filament to provide the area necessary for passing the samequantity of light as the Slits of the more inwardly disposed zones. FIG.3A has the slits all rotated to a position where the center line of theslits lie along a single radius. it is apparent therefrom that theradially extending sides of the slits lie along radii which in each casedefine equal angles with each other with the exception of the slit forthe finest zone. Furthermore, the radial extent of each slit varies toprovide slits having the areas necessary to provide the same quantity oflight when the slits are subjected to full illumination by the lightsource. It will be noted that the slits 33s for the finest zone subtendan arc of a much smaller angle than the other slits and this is due tothe fact that the divisions of the line zone in the itlustrated codewheel are so fine that a larger slit would interfere with the definitionobtained. Accordingly, the slit is made as wide as possible consonantwith the requirements of definition, and the radial extent of the slitis such as to provide the necessary slit area so that the slit passesthe same quantity of light as the other slits.

It will be noted that the radial extents of the zones vary in the samemanner as the slits and decrease proceeding outwardly from the center ofthe code Wheel except for the finest zone. This arrangement enables thediameter of the wheel to be kept at a minimum. The variation in radialextent of the zones is clearly shown in FIG. 2.

As pointed out hereinbefore, the photoelectric cells for reading thevarious zones are not aligned on a single zero reading line but ratherthe zero lines of the various zones are angularly displaced from eachother and the cells are correspondingly displaced so that the photocellsare distributed about the code wheel. This facilitates the use of twocells for each zone in a small space without interfering with thedefinition of the wheel.

While the Gray code has been utilized in the described structure, itwill be understood that certain aspects of the present invention areequally applicable to code wheels coded in other codes, such as thestraight binary code. For example, the use of different size slits andthe inter changing of zones would be useful in a binary coded wheel.

Furthermore, certain aspects of the present invention, such as theinterchanging of the zones of the wheel and the use of a single codezone where the code has two digits represented by code zones with equalnumber of divisions are applicable in encoders using other thanphotoelectric means to read the code on the wheel, for example, the codecould consist of areas which are alternately magnetic and nonmagnetic orelectrically conductive and nonelcctrically conductive.

The preferred embodiment of the present invention has been described asutilizing two photoelectric elements or cells for reading each of thezones which represent in dividual digits of the number representing theangular position of the shaft. The two cells for reading a zone areinterconnected to provide one output level when a transparent area isopposite to one cell and a lower output level when an opaque area isopposite to the same cell. The manner of combining the output of thephotocells to provide a single output signal having a high level when atransparent area is opposite to one of the photocells and a low levelwhen an opaque area is opposite to the same photocell does not, per se,form a part of this invention and has not, therefore, been described.Sutfice it to say, that for the purpose of the present invention, whentwo photocells are utilized for reading a zone, regardless of whether itis in the manner described or for some other reason, such as tocompensate for backlash, the displacing of the Zones so that thephotocells are distributed about the code wheel facilitates themechanical construction of the wheel in compact form. One encodingdevice where two photocells are utilized to read each zone of a codewheel is described in Patent No. 2,779,539 issued to Sidney Darlington.

The code wheel and disk have not been described in detail since thetechniques for making code wheels having alternate opaque andtransparent areas are known and the same techniques may be utilized forconstructing the disk of the present invention. Preferably, the codewheel is a glass code wheel having a photographic surface on one sidethereof which is developed to provide the opaque and transparent areas.This technique of manufacturing a code disk is a known technique. Themask is preferably constructed in the same manner as the code disk.

The preferred and described embodiment of the present invention utilizesphotoelectric cells as light-responsive elements for reading the codewheel. It will be appreciated that other light-responsive elements maybe used. Furthermore, it will be recognized that other radiation whichcan be either blocked or transmitted by a code wheel and mask in thesame manner as light is the equivalent of light insofar as the presentinvention is concerned and a source of such radiation and devicesresponsive to such radiation would be the equivalent of the photocellsand light source shown herein.

While the preferred embodiment of the present invention has beendescribed in considerable detail, it is hereby my intention to cover allmodifications, constructions, and arrangements which are apparent tothose skilled in the art and which fall Within the scope and spirit ofthe present invention.

Having described my invention, I claim:

l. A coding device for digitally indicating shaft position comprising acode wheel having a code thereon in the form of a plurality ofconcentric code zones each comprised of alternate transparent and opaqueareas, said zones being displaced radially from each other and each zoneextending circularly about the axis of said wheel, a plurality of readerunits including a unit for each zone disposed on one side of said wheeland positioned to receive light through the transparent areas of thecorresponding zone as the areas rotate by the unit, said units eachincluding a light-responsive element, a light source mounted on theother side of said wheel and disposed radially inwardly of said wheelfrom said reader units, a light blocking mask adjacent said wheel andblocking light traveling from said source to said reader units, saidmask having a slit therein opposed to each reader unit and adjacent thecorresponding zone to allow the passage of light from the light sourcethrough a transparent area of the zone when passing said slit to saidlight-responsive elements, said slits having areas proportioned suchthat the quantity of light passed by each slit when subjected to fullillumination from said source through a transparent area of said codebeing substantially the same for all of said slits.

2. An encoding device as defined in claim 1 wherein each of said readerunits comprises a light-responsive element disposed to intercept thelight transmitted through the transparent areas of the correspondingzone and the corresponding slit, a member supporting each of said unitswith the optical axes of said elements being inclined with respect tosaid wheel and along the path of the light received by the element fromsaid light source.

3. A coding device as defined in claim 1 wherein said slits have sideslying along radii emanating from the extended axis of said wheel withthe radii for each slit defining a given angle which is the same for aplurality of said slits for different zones and wherein the radialextent of said plurality of slits and the corresponding zones decreasesas one proceeds outwardly from the center of said wheel.

4. An encoding device as defined in claim 3 wherein each of said readerunits comprises a light-responsive ele ment disposed to intercept thelight transmitted through the transparent areas of the correspondingzone and the corresponding slit, a member supporting each of said unitswith the optical axes of said elements being inclined with respect tothe Wheel and lying along the path of the light received by the elementfrom said light source.

5. In a coding device as defined in claim 1 wherein the radial extent ofa plurality of said slits and zones decreases proceeding outwardly fromthe center of said wheel and the circumferential extent of said slitsincreases proceeding outwardly from the center of said wheel.

6. A code wheel as defined in claim 1 wherein said code is the Gray codein which two digits of the Gray numbers are representable by zoneshaving the same number of divisions but which zones are displacedangularly with respect to each other, said code wheel having a singlezone thereon representing both of said digits of the code and twoangularly displaced reader units for said single zone.

7. A coding device as defined in claim 1 wherein more than two of saidcode zones have different numbers of divisions therein and wherein atleast one of said plurality of code zones is disposed between zoneswhich each have a greater number of divisions than said one of saidplurality of code zones, and said reader units each comprises angularlydisplaced reading elements.

8. A code wheel as defined in claim 7 wherein said code is the Gray codein which two digits of the Gray numbers are representable by zoneshaving the same number of divisions but which zones are displacedangularly with respect to each other, said code wheel having a singlezone thereon representing both of said digits of the code and twoangularly displaced reader units for said single zone.

9. A coding device comprising a code wheel having a plurality ofconcentrically arranged circular code zones thereon with each zonecomprising alternate transparent and opaque areas, a light sourcedisposed on one side of said zones and inwardly of said wheel withrespect to said zones, a light-responsive reader unit for reading eachof said zones, supported adjacent the other side of said wheel adjacentthe zone read by the unit, said units comprising a light-responsiveelement, a light mask mounted adjacent said wheel and having a slitcorresponding to each of said zones and adapted to pass light from saidsource to said element of said corresponding reader unit for the zonewhen a transparent area of the zone is moving past the slit and readerunit, said slits increasing in circumferential extent proceedingoutwardly from the center of said wheel and said zones and slitsdecreasing in radial extent proceeding outwardly from the center of saidwheel.

10. A code wheel as defined in claim 9 wherein said code is the Graycode in which two digits of the Gray numbers are representable by zoneshaving the same number of divisions but which zones are displacedangularly with respect to each other, said code wheels having a singlezone thereon representing both of said digits of the code and twoangularly displaced reader units for said single zone.

11. A coding device comprising a code Wheel having a plurality ofconcentrically arranged circular code zones thereon with each zonecomprising alternate transparent and opaque areas, a light sourcedisposed on one side of said zones and inwardly of said wheel withrespect to said zones, a respective light-responsive reader unit forreading each of said zones supported adjacent the other side of saidwheel adjacent the zone read by the unit, a light mask mounted adjacentsaid wheel and having a slit corresponding to each of said zones anddisposed intermediate the reader unit for the zone and the wheel andadapted to pass light transmitted by a transparent area of the zone tothe reader unit, said slits increasing in circumferential extentproceeding outwardly from the center of said wheel and said zones andslits decreasing in radial extent proceeding outwardly from the centerof said wheel, said slits having sides lying substantially alonge radiiemanating from the extended axis of said wheel with the angle defined bythe radii for each of said slits being substantially the same as theangle defined by the radii of the other slits.

12. A coding device comprising a member having a code thereon in theform of opaque and transparent areas, said code lying in a plane, alight source on one side of said code and positioned such that lighttraveling from said source through a transparent area of said codetravels a path inclined with respect to said plane, a reader unitcomprising a light-responsive element supported on the a other side ofsaid member and adapted to receive light from said source when atransparent area is opposite to said element, a support member mountingsaid unit with the optical axis thereof along the path for light fromsaid source through a transparent area to said unit.

13. A code wheel as defined in claim 11 wherein said code is the Graycode in which two digits of the Gray numbers are representable by zoneshaving the same number of divisions but which zones are displacedangularly with respect to each other, said code wheel having a singlezone thereon representing both of said digits of the code and twoangularly displaced reader units for said single zone.

14. A coding device for digitally indicating angular position by anumber having a plurality of digits therein and representable bydivisions of a circular zone on a code wheel, certain digits of saidnumber being representable by code zones having the same number ofdivisions but angularly displaced from one another, a code wheel havinga plurality of concentric code zones thereon representing said digitsand including a single code zone representing said certain digits andreader units corresponding in number to said certain digits for readingsaid single code zone, said units being angularly displaced from oneanother.

References Cited UNITED STATES PATENTS 3,218,626 11/1967 Schuman 340-347FOREIGN PATENTS 932,135 8/1960 Great Britain.

MAYNARD R. WILBUR, Primary Examiner. G. R. EDWARDS, Assistant Examiner.

1. A CODING DEVICE FOR DIGITALLY INDICATING SHAFT POSITION COMPRISING ACODE WHEEL HAVING A CODE THEREON IN THE FORM OF A PLURALITY OFCONCENTRIC CODE ZONES EACH COMPRISED OF ALTERNATE TRANSPARENT AND OPAQUEAREAS, SAID ZONES BEING DISPLACED RADIALLY FROM EACH OTHER AND EACH ZONEEXTENDING CIRCULARLY ABOUT THE AXIS OF SAID WHEEL, A PLURALITY OF READERUNITS INCLUDING A UNIT FOR EACH ZONE DISPOSED ON ONE SIDE OF SAID WHEELAND POSITIONED TO RECEIVE LIGHT THROUGH THE TRANSPARENT AREAS OF THECORRESPONDING ZONE AS THE AREAS ROTATE BY THE UNIT, SAID UNITS EACHINCLUDING A LIGHT-RESPONSIVE ELEMENT, A LIGHT SOURCE MOUNTED ON THEOTHER SIDE OF SAID WHEEL AND DISPOSED RADIALLY INWARDLY OF SAID WHEELFROM SAID READER UNITS, A LIGHT BLOCKING MASK ADJACENT SAID WHEEL ANDBLOCKING LIGHT TRAVELING FROM SAID SOURCE TO SAID READER UNITS, SAIDMASK